The goal of the proposed research is to understand a form of genomic instability that we believe is relevant to cancer. Certain chromosomal rearrangements show delayed replication timing (DRT) of the entire affected chromosome and delayed mitotic condensation (DMC) in the following mitosis. Chromosomes with the DRT/DMC phenotype are found in a variety of human primary tumors and tumor cell lines, as well as primary human cells exposed to ionizing radiation. DRT/DMC chromosomes are associated with karyotypic abnormalities and secondary rearrangements involving the DRT/DMC chromosome, characteristics which are associated with cancer cells. Therefore, chromosomes with DRT/DMC may be an important source of genomic instability in normal cells exposed to ionizing radiation and in human cancer. Using chromosome engineering, we can reproducibly generate balanced translocations that show DRT/DMC in vitro. Our data suggest that the functional disruption of loci controlling the replication timing of entire chromosomes results in the DRT/DMC phenotype. In this application, I propose a somatic cell genetic approach to determine how DRT/DMC is regulated, and to identify and define specific chromosomal loci that segregate with the DRT/DMC phenotype. Specifically, I propose to 1) determine if the DRT/DMC phenotype is dominant or recessive by generating new translocations with existing loxP tagged chromosomes, 2) characterize the insertion sites in our existing engineered chromosomes that result in DRT/DMC, and 3) characterize the cis-acting elements responsible for regulating DRT/DMC. The ultimate goal of this proposal is to determine the molecular basis for this DRT/DMC phenotype. [unreadable] [unreadable] The effective prevention and treatment of cancer is of immense importance to public health. We are working to understand why damage to a small portion of the genetic material sometimes causes a loss of stability across the entire genome, which may lead to cancer. Understanding how this instability occurs may one day lead to the prevention of cancer at an early stage. [unreadable] [unreadable] [unreadable]